Road surface discriminator and apparatus applying same

ABSTRACT

Light is projected upon a road surface (LD) from a light source (11) for illuminating the road surface. Diffuse reflected light from the road surface (LD) is received by light sensors (31A, 31B) through a spatial filter, and the resulting light-reception signals are applied to a differential amplifier circuit (51). A center-frequency component corresponding to the spatial frequency of the spatial filter is extracted by a tracking band-pass filter (52), and the intensity (Da) thereof is detected by an amplitude detector circuit (54). Low-frequency component intensity (Db) corresponding to a spatial frequency lower than the spatial frequency of the spatial filter is detected by a tracking low-pass filter (55) and an amplitude detector circuit (56). The condition of the road surface is judged to be snow, gravel or asphalt in a discriminating circuit (60) based upon the center-frequency component intensity (Da) and the low-frequency component intensity (Db).

TECHNICAL FIELD

This invention relates to a discriminator used upon being installed in avehicle to discriminate the condition (snow-covered, gravel-covered,asphalt, wet, frozen, etc.) of the surface of a road on which thevehicle travels, and to an apparatus to which the discriminator isapplied.

BACKGROUND ART

An antilock brake system (ABS) (or antiskid system) is a system forshortening vehicle braking distance as much as possible by controlling abrake so as to obtain a slip factor that maximizes braking force. Sincea slip factor for maximizing braking force differs depending upon thecondition of the road surface, it is necessary that the condition of theroad surface be discriminated so as to carry out optimum ABS controlsuited to the conditions of the road surface. In addition, it isnecessary to identify very bumpy gravel roads in order to absorbunpleasant vibration and maintain ideal riding comfort by controllingthe suspension in conformity with the roughness of the road surface.

One technique for optically discriminating road surface conditionswithout contact is disclosed in "Road-Surface Condition Sensor UtilizingPolarizing Characteristic of Road Surface Reflection", by ToshioTakehana, Hikari Gijutsu Contact, Vol. 27, No. 3 (1989), pp. 158-164.According to this technique, a light-emitting element and alight-receiving element are arranged in such a manner that angle ofincidence and angle of reflection take on the value of the Brewsterangle (53°). This utilizes the fact that since a wet road surfaceapproximates a mirror surface, the degree of polarization approachesunity, whereas in the case of a dry road surface, the degree ofpolarization approaches zero.

However, this technique is such that an asphalt road surface isidentified as being in either a wet condition or a dry condition; gravelroads and snow-covered roads cannot be discriminated. Further, since theangles at which the light-emitting element and light-receiving elementare arranged are determined by the Brewster angle, precise positioningis required and the light-transmitting and light-receiving elements mustbe provided quite far apart.

Further, road-surface condition estimation suited for application toantiskid control is available ("Road-Surface Estimation Using FuzzyLogic", by Katsuhiro Oba et. al., Jidosha Gijutsukai, CorporateJuridical Person, Gakujutsu Koenkai Zensatsushu 881, May, 1988, 881028).However, since this technique is suitable for application to an antiskidsystem, it lacks universality.

DISCLOSURE OF THE INVENTION

The present invention provides a road surface discriminator capable ofbeing expanded to discriminate road surface conditions of a wide varietyof types, thereby making many applications possible.

The basic structure of the road surface discriminator according to thepresent invention is as follows: Specifically, the road surfacediscriminator comprises a first light source for projecting light towarda road surface; a first spatial filter light-receiving optical systemhaving first spatial filter means and first light-receiving means,wherein reflected light from the road surface on which light isprojected from the first light source is received by the firstlight-receiving means through the first spatial filter means and thefirst-light receiving means outputs an electric signal representing thereflected light received; a second light-receiving optical system havingsecond light-receiving means, Wherein reflected light from the roadsurface on which light is projected from the first light source isreceived by the second light-receiving means and the second-lightreceiving means outputs an electric signal representing the reflectedlight received; a first signal processing circuit for detectingintensity a center-frequency component, which corresponds to the spatialfrequency of the first spatial filter means, from the electric signaloutputted by the first spatial filter light-receiving optical system; asecond signal processing circuit for detecting intensity of alow-frequency component, which corresponds to a spatial frequency lowerthan the spatial frequency of the first spatial filter means, from theelectric signal outputted by the second light-receiving optical system;and discriminating means for discriminating conditions of the roadsurface based upon the intensity of the center-frequency componentdetected by the first signal processing circuit and the intensity of thelow-frequency component detected by the second signal processingcircuit.

Preferably, the first light source, the first spatial filterlight-receiving optical system and the second light-receiving opticalsignal are arranged so that the first- and second-light receiving meansreceive diffusely reflected light from the road surface.

In a first embodiment of the invention, the second light-receivingoptical system is contained in the first spatial filter light-receivingoptical system and the second light-receiving means is the firstlight-receiving means. The electric signal outputted by the firstlight-receiving means is applied to the first signal processing circuitand the second signal processing circuit.

In a second embodiment, part of the second light-receiving opticalsystem is contained in the first spatial filter light-receiving opticalsystem and the second light-receiving means receives reflected lightthat does not pass through the first spatial filter means.

In a third embodiment, the second light-receiving optical system has asecond spatial filter having a spatial frequency lower than the spatialfrequency of the first spatial filter means. The second light-receivingmeans receives reflected light through the second spatial filter.

The spatial frequency characteristic (especially of diffused light)exhibited by a road surface has a distinct feature in the spatialfrequency spectrum. Specifically, intensity is high in the region of lowspatial frequency; the higher the spatial frequency, the lower theintensity. The inventors have discovered that the intensity differs independence upon the road surface conditions in this low-frequency regionof high intensity. In other words, the inventors have found that theroad surfaces conditions are ranked as follows in order of decreasingintensity: snow, gravel (earth, sand) and asphalt (concrete).

The spatial frequency (center frequency component) of the first spatialfilter optical system is set to a comparatively high portion of thedetectable spatial frequency region. In a region of frequency lower thanthe center frequency, the spatial frequency of the low-frequencycomponent extracted is set to a portion in which intensity will vary asmuch as possible in dependence upon the road surface conditions.

The intensity of the low-frequency component is standardized(normalized) by the intensity of the center frequency component. As aresult, the cause of a fluctuation in amount of light and of afluctuation in reflectivity, etc., is eliminated. By comparing thestandardized low-frequency component intensity with a predeterminedthreshold value, at least one of snow, gravel and asphalt can bediscriminated.

The condition of wetness of a road surface, especially of an asphaltroad surface, is discriminated based upon regularly reflected light.

In this case, a second light source for projecting light toward the roadsurface and a third light-receiving optical system, which includes athird light-receiving element, are provided. The second light source andthird light-receiving optical system are arranged in such a manner thatthe third light-receiving element receives regular reflected light fromthe road surface on which light is projected from the second lightsource. The discriminating means discriminates the condition of wetnessof the road surface based upon an output signal from the thirdlight-receiving element.

In a preferred embodiment, a light-quantity detector for detecting thequantity of light in the projected light from the second light source isfurther provided. The discriminating means discriminates the conditionof wetness of the road surface based upon a value obtained bynormalizing the output signal of the third light-receiving element bythe quantity of light detected by the light-quantity detector. As aresult, the adverse effects of a fluctuation in the quantity of light ofthe second light source can be eliminated.

In another preferred embodiment, the third light-receiving opticalsystem is contained in the first spatial filter light-receiving opticalsystem. The third light-receiving element is the first light-receivingelement.

In this case, separating means is provided for separating an outputsignal of the first light-receiving element into a first signalcomponent, which results from the projected light of the first lightsource, and a second signal component, which results from the projectedlight of the second light source. The first signal processing circuitdetects the intensity of the center-frequency component of diffusedlight based upon the first signal component separated by the separatingmeans. A third signal processing circuit is provided for detecting theintensity of the center-frequency component of regular reflected lightbased upon the second signal component separated by the separatingmeans.

The discriminating means discriminates the condition of wetness of theroad surface based upon a ratio of the intensity of the center-frequencycomponent of regular reflected light to the intensity of thecenter-frequency component of diffused light. This ratio becomes fairlylarge in the case of a wet road surface, so that an accurate judgmentcan be expected.

The separation of the above-mentioned signals can be implemented in thefollowing manner: A drive circuit is provided for driving the firstlight source and the second light source based upon respective signalsof different phase or different frequency. The separating means isimplemented by a demodulating circuit for demodulating respectivesignals that have been modulated by signals of different phase ordifferent frequency.

The fact that a road surface is frozen also can be determined. Atemperature sensor for measuring road surface temperature or airtemperature is provided. When the discriminating means has discriminatedwetness, the discriminating means determines that the road surface isfrozen in a case where the temperature sensed by the temperature sensoris less than a predetermined value.

A snow-covered road also can be judged by a diffused light component.Specifically, a fourth signal processing circuit is provided fordetecting a diffused light component of very low frequency based uponthe output signal of the first light-receiving element or secondlight-receiving element. The discriminating means judges that a roadsurface is snowy in a case where the diffused light component detectedby the fourth signal processing circuit exceeds a predetermined value.

Preferably, the first light source is composed of a plurality oflight-emitting elements and a plurality of lenses corresponding to thelight-emitting elements and disposed in front of them. The optic axis ofeach light-emitting element and the optical axis of the lenscorresponding to it are so disposed as to be offset. As a result, theprojected light of the first light source is prevented from spreading.

The first signal processing circuit includes a tracking band-pass filteras one example. The tracking band-pass filter outputs a signalrepresenting ground speed. The pass band of the tracking band-passfilter is controlled based upon the speed signal. The road surfacediscriminator is capable of detecting ground speed as well.

The second signal processing circuit includes, as one example, atracking low-pass filter in which cut-off frequency is controlled basedupon the speed signal.

The present invention provides a road surface discriminator having amore simplified structure (especially a more simplified electricalconstruction). The road surface discriminator comprises a light sourcefor projecting light toward a road surface; a spatial filterlight-receiving optical system for receiving diffuse reflected lightfrom the road surface, on which light is projected from the lightsource, through a spatial filter and outputting an electric signalrepresenting the reflected light received; a signal processing circuitfor detecting a center-frequency component, which corresponds to thespatial frequency of the spatial filter, based upon the electric signaloutputted by the spatial filter light-receiving optical system; a firstcounter for counting zero cross points of the center-frequency componentoutputted by the signal processing circuit; a second counter forcounting zero cross points of the electric signal outputted by thespatial filter light-receiving optical system; and discriminating meansfor discriminating condition of the road surface based upon a countvalue in the first counter and a count value in the second counter.

At least one of snow, gravel and asphalt is discriminated by thediscriminating means in the above structure.

According to one embodiment, the discriminating means performsdiscrimination and the first and second counters start counting atprescribed times. According to another embodiment, the discriminatingmeans performs discrimination and the first and second counters startcounting whenever the count in the first counter reaches a prescribednumber.

According to yet another preferred embodiment, the first and secondcounters are provided in a plurality of pairs. The moment at whichcounting by the first and second counters starts is set so as to bedifferent for each pair. As a result, accuracy can be maintained and thediscrimination period can be shortened.

The simplified road surface discriminator described above also can be sodeveloped as to be capable of discriminating wetness, freezing, etc.

In all of the road surface discriminators described above, the followingarrangement is recommended in order to prevent the diffuse reflectedlight from becoming so large as to saturate the processing circuit incase of snow:

Specifically, there are provided an amplifier circuit for amplifyingelectric signals inclusive of light-reception signals, and means forcontrolling the amplification factor of the amplifier circuit independence upon result of discrimination performed by the discriminatingcircuit.

Alternatively, control means is provided for varying the quantity ofprojected light from the first or second light source in dependence uponresult of discrimination performed by the discriminating circuit.

This invention also can be developed to discriminate not only a roadsurface but also the surface of an object.

The present invention is further applicable to a vehicle slippingwarning, to control of braking or acceleration for antiskid purposes,and to vehicle suspension control.

The present invention provides a vehicle or mobile body having a roadsurface discriminator.

Other features of the present invention will become more apparent in thedescription of embodiments with reference being made to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 3 illustrate the optical structure (a first aspectthereof) of a road surface discriminator, in which FIG. 1 is aperspective view, FIG. 2 a longitudinal sectional view and FIG. 3 afront view of an optical system for regular reflected light;

FIGS. 4 and 5 schematically illustrate an example of the structure of alight source for illuminating the surface of a road, in which FIG. 4 isa plan view and FIG. 5 a sectional view;

FIG. 6 is a sectional view showing a specific example of a light sourcefor illuminating a road surface;

FIG. 7 is a graph showing results of actual measurement;

FIG. 8 is a block diagram illustrating the electrical configuration (afirst aspect thereof) of the road surface discriminator;

FIG. 9 is a circuit diagram showing a specific example of a differentialamplifier circuit;

FIG. 10 is a circuit diagram showing a specific example of a trackingband-pass filter;

FIG. 11 is a block diagram showing a specific example of an amplitudedetector circuit;

FIG. 12 is a flowchart illustrating a road-surface judgment algorithm (afirst aspect thereof);

FIG. 13 is a flowchart illustrating a road-surface judgment algorithm (asecond aspect thereof);

FIG. 14 is a flowchart illustrating a road-surface judgment algorithm (athird aspect thereof);

FIG. 15 is a flowchart illustrating a road-surface judgment algorithm (afourth aspect thereof);

FIG. 16 is a block diagram illustrating the electrical configuration (asecond aspect thereof) of the road surface discriminator;

FIG. 17 is a block diagram illustrating the electrical configuration (athird aspect thereof) of the road surface discriminator;

FIG. 18 is a block diagram showing an example of the construction of adualcomb filter;

FIG. 19 is a waveform diagram of an input signal and FIG. 20 is anenlarged view of a portion thereof;

FIG. 21 is a block diagram illustrating the electrical configuration (afourth aspect thereof) of the road surface discriminator;

FIG. 22 is a block diagram illustrating the electrical configuration (afifth aspect thereof) of the road surface discriminator;

FIG. 23 is a perspective view illustrating the optical structure (asecond aspect thereof) of the road surface discriminator, and FIG. 24 isa front view thereof;

FIG. 25 is a block diagram illustrating the electrical configuration (asixth aspect thereof) of the road surface discriminator;

FIG. 26 is a front view illustrating the optical structure (a thirdaspect thereof) of the road surface discriminator;

FIG. 27 is a block diagram illustrating the electrical configuration (aseventh aspect thereof) of the road surface discriminator;

FIG. 28 is a flowchart illustrating a road-surface judgment algorithm (afifth aspect thereof);

FIG. 29 is a flowchart illustrating a road-surface judgment algorithm (asixth aspect thereof);

FIG. 30 is a flowchart illustrating a road-surface judgment algorithm (aseventh aspect thereof);

FIG. 31 is a block diagram illustrating the electrical configuration (aneighth aspect thereof) of the road surface discriminator;

FIG. 32 is a waveform diagram showing a signal in case of a snow-coveredroad;

FIG. 33 is a waveform diagram showing a signal in case of a gravel road;

FIG. 34 is a block diagram illustrating the electrical configuration (aninth aspect thereof) of the road surface discriminator;

FIG. 35 is a block diagram illustrating the electrical configuration (atenth aspect thereof) of the road surface discriminator;

FIG. 36 is a time chart showing the operation of a counter;

FIG. 37 is a block diagram illustrating the electrical configuration (an11th aspect thereof) of the road surface discriminator;

FIG. 38 is a block diagram illustrating the electrical configuration (an12th aspect thereof) of the road surface discriminator;

FIG. 39 illustrates an apparatus which issues a warning in dependenceupon the results of road surface discrimination;

FIG. 40 illustrates an antilock brake system (ABS);

FIG. 41 illustrates a table of optimum slip factors;

FIG. 42 illustrates a vehicle in which an ABS has been installed;

FIG. 43 illustrates a vehicle in which a suspension control apparatushas been installed; and

FIG. 44 is a sectional view of a shock absorber.

BEST MODE FOR CARRYING OUT THE INVENTION

A road surface discriminator generally is installed in a vehicle. Lightfrom the optical system of the road surface discriminator is projectedtoward the surface of a road and light reflected from the road surfaceis received by the optical system. The surface condition of the road isdiscriminated by a signal processing circuit based upon an electricsignal obtained from the optical system.

Typical examples of road surface conditions identified in thisembodiment are as follows:

snow;

asphalt (or concrete); and

gravel (earth (soil) or sand).

In this embodiment, whether or not a road surface is frozen also isdiscriminated.

Furthermore, according to this embodiment, an asphalt (concrete) roadsurface can subdivided into the following two conditions:

wet asphalt (concrete) and

dry asphalt (concrete).

Accordingly, the modes of discrimination include identifying any oneroad surface condition among the above-mentioned road surface conditionsand distinguishing between any two or more road surface conditions.Representative examples of modes of discrimination are as follows:

a. identification of a snowy road;

b. identification of an asphalt road (concrete road);

c. identification of a gravel road (earth or sandy road);

d. identification of a frozen road surface;

e. identification of a wet asphalt road;

f. identification of a dry asphalt road;

g. distinguishing between a snowy road and an asphalt road;

h. distinguishing between a snowy road and a gravel road;

i. distinguishing between an asphalt road and

a gravel road;

j. distinguishing among a snowy road, an asphalt road and a gravel road;

k. discriminating whether an asphalt road is in a wet condition or a drycondition in case of g, i and j above; and

m. discriminating whether a road surface is frozen or not in case of g,h, i, j and k above.

In the embodiments illustrated below, the description is focused on themode m, which has the most types of road surface conditions to bediscriminated. It goes without saying that a road surface discriminatorapparatus and method in which road surface discrimination of any modeamong a˜k above is possible can be realized by extracting solely therequired portion of the optical structure, the required portion of theelectrical configuration and the required portion of an algorithm.

(1) Optical structure (first aspect thereof) of road surfacediscriminator

FIGS. 1 through 3 illustrate a first example of the optical structure ofa road surface discriminator. In order to reduce the number of drawings,all optical elements necessary in the optical system to actually executeall of several road surface discrimination algorithms described indetail later are drawn. Conversely speaking, this optical system alsoincludes optical elements not necessary for the purpose of executing acertain road surface discrimination algorithm. It can be said that FIGS.1 through 3 express all optical elements contained in the opticalsystems of several road surface discriminators. This fact applies to thesignal processing circuit of FIG. 8 as well. Accordingly, when thisoptical system and the signal processing circuit shown in FIG. 8 areused, road surface discrimination of the above-mentioned mode m ispossible. In a case where a road surface discriminator capable of roadsurface discrimination of any of the above-mentioned modes a˜k isimplemented, unnecessary optical elements and electrical circuitelements need only be deleted.

A light source 11 for road surface illumination and a light source 12for regular (specular) reflected light are included in the opticalsystem. The light sources 11 and 12 are constituted by light-emittingdiodes. An example of a preferred construction for the light source 11for road surface illumination will be described later. The light source11 for road surface illumination projects light obliquely downward inthe traveling direction of the vehicle. The light source 12 for regularreflected light projects light obliquely downward in a directionorthogonal to the traveling direction. Preferably, the wavelengths ofthe light projected from the light sources 11 and 12 differ. As aresult, the light of these light sources reflected from a road surfaceLD (the road also is represented by LD) can be separated by an opticalfilter.

A light-receiving optical system for receiving diffuse reflected lightfrom the road surface includes a light-receiving lens 21, a slit plate22 and a collimating lens 24. The focal point of the light-receivinglens 21 and the focal point of the collimating lens 24 are at the sameposition, and a slit (diaphragm) 22a in the slit plate 22 is situated atthese focal points. The slit 22a extends in a direction perpendicular tothe vehicle traveling direction. Such an optical system is referred toas a telecentric optical system. That is, of the reflected light fromthe road surface LD, only light rays perpendicular to the road surfaceLD and parallel to one another (in FIG. 2) converge at the focal pointof the light-receiving lens 21 and pass through the slit 22a. Light raysthat have passed through the slit 22a are rendered parallel by thecollimating lens 24. The light from the light source 11 impingesobliquely on the road surface LD. Only light reflected perpendicularlyfrom the light source LD passes through the slit 22a. Thus, only diffusereflected light from the road surface LD is collimated by thecollimating lens 24 and enters a spatial filter optical system. (Thatis, regular (specular) reflected light from the road surface LD does notenter the spatial optical system.)

Preferably, an optical filter 23 is placed at the position of the slit22a of slit plate 22. This filter 23 exhibits wavelength selectivity forpassing only the light projected from the light source 11 forilluminating the road surface. As a result, light from the light source12 for regular reflected light and other extraneous light (sunlight,light from road illuminating lamps, etc.) is prevented from impingingupon the spatial filter optical system. The light projected from thelight source 11 preferably is infrared light.

The spatial filter optical system includes a grating plate (slit array)25, a prism array 26, a condenser lens 27 and two light sensors(light-receiving elements, e.g., photodiodes or phototransistors) 31A,31B. The prism array 26 basically acts as a spatial filter.

The prism array 26 is constituted by a number of prisms. The prisms arearrayed in the traveling direction of the vehicle and extend in adirection perpendicular to the traveling direction. The prism array 26preferably is molded as a unitary body. The light rays rendered parallelby the collimating lens 24 are separated by refraction alternately backand forth (with the traveling direction serving as the reference) eachat a fixed pitch width by the prisms of the prism array 26. Theseparated light rays are each condensed by the condenser lens 27 andimpinge upon the two light sensors 31A, 31B.

The light indicated by the dots-and-dashes lines in FIG. 2 impinges uponthe light sensor 31A, and the light indicated by the dots lines impingesupon the light sensor 31B. The width these light rays depends upon theperiod at which the prisms are arrayed. The prism array perioddetermines the characteristic (period) of the spatial filter.

A number of slits arrayed in the traveling direction of the vehicle andextending in the direction perpendicular to the traveling direction areformed in the grating plate (slit array) 25. The period at which theseslits are arrayed is half the array period of the prisms of prism array26. Of the light rendered parallel by the collimating lens 24, the lightwhich has passed through the slits impinges upon the prism array 26 soas to be separated, as described above, and the separated light isreceived by the light sensors 31A, 31B alternately in space. The gratingplate 25 prevents stray light from impinging upon prism array 27.

The light sensors 31A and 31B are arranged in spaced relation in thedirection of vehicle travel. The spacing is decided by the period of theprisms in the prism array 26 and the magnification of the condenser lens27. Mirrors 28 are provided on both sides of the light sensors 31A and31B and act in such a manner that light not condensed on thelight-receiving surfaces of the light sensors 31A, 31B by the lens 27will impinge upon the light sensors 31A, 31B as much as possible.

As will be illustrated later, the output signals from the two lightsensors 31A and 31B are applied to a differential amplifier circuit andthe difference between them is calculated. The output signal of thedifferential amplifier circuit contains a frequency component (which isdependent upon the speed of the vehicle) corresponding to a spatialfrequency component representing the condition of the road surface thatcauses a fluctuation in diffuse reflected light, wherein the roadsurface condition includes roughness of the road surface.

The light rays incident upon the light sensor 31A and the light raysincident upon the light sensor 31B are out of phase by a period which ishalf the spatial period selected by the spatial filter. Accordingly, thespatial center frequency component is doubled by taking the differencebetween the output signals of the two light sensors 31A and 31B.Direct-current (DC) components are canceled out mainly by thisdifferential processing.

The light source 12 for regular reflected light and a light sensor 32for sensing regular reflected light are arranged in a planeperpendicularly intersecting the traveling direction of the vehicle insuch a manner that the angle of incidence of projected light from thelight source 12 upon the road surface LD and the angle of reflection ofreflected light from the road surface incident upon the light sensor 32are made equal. Since the angle of incidence and the angle of reflectioncan be made smaller than the Brewster angle (53°), a reduction in thesize of the optical system can be expected. Preferably, an opticalfilter which allows passage solely of light having the wavelength of theprojected light from the light source 12 and a condenser lens are placedin front of the light sensor 32.

A road surface thermometer 33 measures the temperature of the roadsurface and is implemented by an infrared radiation thermometer, by wayof example. The road surface thermometer 33 need not be included in theoptical system and may be provided at another suitable location in thevehicle.

Furthermore, a light sensor 34 for monitoring the amount of projectedlight is provided to receive part of the projected light from the lightsource 12 for regular reflected light. The light sensor 34 will bedescribed later.

FIGS. 4 and 5 illustrate an example of the construction of the lightsource 11 for illuminating the road surface. The light source 11 isconstituted by light-emitting elements (e.g., light-emitting diodes) 11aarrayed in two dimensions, and a lens array 13 having a number of convexlenses 13a arrayed in two dimensions to condense and then project lightfrom the light-emitting elements 11a. One lens 13a corresponds to onelight-emitting element 11a. The optic axes in pairs of thelight-emitting elements and lenses situated in the vicinity of thecenter substantially coincide. However, the closer the position is tothe periphery, the more the optic axes of the light-emitting elements11a and the optic axes of the corresponding lenses 13a are offset fromeach other so as to make the light condensed and projected by the lenses13a come as close to the center as possible. As a result, the lightprojected from the light source 11 does not spread significantly butdiverges to an area that is as small as possible to illuminate the roadsurface LD. In other words, the light can be utilized more efficientlyand it is possible to reduce the number of light-emitting elementsconstructing the light source 11.

FIG. 6 illustrates the road surface illuminating light source 11 in aform closer to that used in actuality. The multiplicity oflight-emitting elements 11a are mounted on a printed circuit board 14and connected to a wiring pattern on the board 14. The light-emittingelements 11a are fitted into recesses in a holder 15 secured to theboard 14. Light shielding plates 16 are provided so as to cover theperiphery of the light-emitting elements 11a (with the exception of theforwardly facing parts thereof). The lens array 13 is placed in front ofthe light-emitting elements 11a and is fixed to the light shieldingplates 16. The optic axes of the light-emitting elements 11a and theircorresponding lenses 13a are offset from each other.

(2) Principles of road surface discrimination

FIG. 7 illustrates an example of actual measurement of a spatialfrequency spectrum represented by a differential signal between theoutput signal of the light sensor 31A and the output signal of the lightsensor 31B. This graph is the result of actually measuring three typesof road surface conditions, namely snowy roads, gravel roads and asphaltroads.

The frequency (electrical center frequency) f of a center frequencysignal component contained in the differential signal between theoutputs of the light sensors 31A and 31B is represented by the productof a spatial center frequency μ, which is selected by the constructionof the spatial filter, and the speed v of the vehicle, as follows:

    f=μ×v                                             Eq. (1)

The spatial center frequency μ is uniquely decided by the constructionof the spatial filter. The road surface period (the period of the roadsurface condition that causes a change in the diffuse reflected light,wherein the road surface condition includes roughness of the roadsurface) selected by the spatial filter is set here at 4 (mm). FIG. 7shows the result of subjecting an electric signal acquired by actualmeasurement to a Fourier transform (FFT: fast Fourier transform) so asto obtain a frequency spectrum, and then normalizing the frequencyspectrum by the spatial center frequency μ. Further, data regardingsnow, gravel and asphalt is normalized in such a manner that the peakvalues (intensities) in the spatial center frequency μ will coincide.

As will be understood from the graph, there are large and distinctdifferences between asphalt, snowy and gravel roads in terms of theintensities of spatial frequency components (in the band below μ/4, forexample) lower than the spatial center frequency μ. These differencesare on the order of one place (factor of ten) or more. The lower thespatial frequency, the larger the difference in intensity in the threetypes of road surface conditions.

Accordingly, it is possible to distinguish among a snowy road, a gravelroad and an asphalt road based upon a value [referred to as"(low-frequency component intensity/center-frequency componentintensity)"=Db/Da] obtained by normalizing the low-frequency componentintensity (at a frequency of μ/4 or μ/10, for example) of the spatialfrequency by the center-frequency component intensity. Threshold valuesTH1, TH2 used to distinguish among these roads should be decided asvalues intermediate the value Db/Da. In FIG. 7, a road is discriminatedto be a snowy road if the value Db/Da is greater than the thresholdvalue TH1, a gravel road if the value Db/Da lies between the thresholdvalues TH1 and TH2 and an asphalt road if the value Db/Da is less thanthe threshold value TH2.

The snowy road mentioned here does not refer to a road covered with newsnow (the entire surface of which is pure white) but rather to a snowsurface worn down by vehicular and pedestrian traffic so as to take on acomparatively bumpy condition (i.e., considerably more bumpy than agravel road, which road condition causes a change in the amount ofdiffuse reflected light).

An earthen or sandy road in which gravel is mixed also exhibits the sametendency, and the frequency spectrum of a concrete road is substantiallythe same as the spectrum of an asphalt road.

(3) Electrical configuration (first aspect) thereof) of road surfacediscriminator

FIG. 8 illustrates an example of the configuration of a signalprocessing circuit included in the road surface discriminator.

The output signals of the light sensors 31A and 31B are applied to adifferential amplifier circuit 51, which outputs a signal representingthe difference between these two signals.

An example of the constructions of the light sensors 31A, 31B anddifferential amplifier circuit 51 are illustrated in FIG. 9. The lightsensors 31A and 31B are each constituted by a photodiode, with thephotodiodes being serially connected. The differential amplifier circuit51 is constituted by an operational amplifier 51A having a feedbackresistor R. The difference between a current I1 which flows into thephotodiode 31A and a current I2 which flows into the photodiode 31B iscalculated at the node of the photodiodes and the difference currententers the operational amplifier 51A. The operational amplifier 51Aconverts the input difference current into a voltage signal V₀ anddelivers this signal. The output voltage V₀ is given by the followingequation:

    V.sub.0 R(I.sub.2 -I.sub.1)                                Eq. (2)

The output voltage V₀ of the differential amplifier circuit 51 isapplied to a tracking band-pass filter [tracking BPF (C)] 52 and atracking low-pass filter [tracking LPF (L)] 55.

The output signal of the tracking BPF 52 is applied to afrequency/voltage (F/V) converter circuit 53. The output signal of theF/V converter circuit 53 represents the speed (ground speed) v of thevehicle in which the road surface discriminator is installed. The outputof the F/V converter circuit 53 is fed back to the tracking BPF 52 andtracking LPF 55 and is used to change the cut-off frequencies (frequencybands) of these filter circuits to follow up the vehicle speed v.

The output signal of the tracking BPF 52 enters an amplitude detectorcircuit 54 as well. The amplitude detector circuit 54 outputs a signalrepresenting the center-frequency component intensity Da describedabove.

The output signal of the tracking LPF 55 enters an amplitude detectorcircuit 56. The amplitude detector circuit 56 outputs a signalrepresenting the low-frequency component intensity Db described above.

An example of the construction of the tracking BPF 52 is illustrated inFIG. 10. The tracking BPF 52 includes a high-pass filter (HPF) and alow-pass filter (LPF) connected serially via a buffer amplifier 75. TheHPF comprises a condenser 71 and a voltage-controlled variableresistance element 73. The LPF comprises a capacitor 72, avoltage-controlled variable resistance element 74. Thevoltage-controlled variable resistance elements 73, 74 are constitutedby FETs, by way of example. A control voltage from a control voltagegenerating circuit 76 is applied to the elements 73, 74 and theresistance values of the elements 73, 74 vary in conformity with thecontrol voltage. A change in the resistance values of the elements 73,74 brings about a change in the cut-off frequencies of the HPF and LPF.The pass band of the tracking BPF 52 is the band between the cut-offfrequency of the HPF and the cut-off frequency of the LPF (the lattercut-off frequency being higher than that of the HPF). The controlvoltage generating circuit 76 generates a control voltage commensuratewith the output voltage signal (which represents the vehicle speed v) ofthe F/V converter circuit 53.

If, by way of example, the period (of the roughness) of the road surfaceselected by the spatial filter in the above-described optical system is5 (mm), then the spatial center frequency μ will be 0.2 (mm⁻¹). Let v(Km/h) represent the speed (ground speed) of the vehicle.

    v(Km/h)=1000 v/3.6 (mm/s)                                  Eq. (3)

The center frequency f of the electric signal obtained from thedifferential amplifier circuit 51 is as follows, based upon Equation(1):

    f=μ×v=200 v/3.6 (Hz)                              Eq. (4)

Accordingly, it will suffice to set the center frequency of the passband of tracking BPF 52 to the frequency indicated by Equation (4) andto vary the center frequency in dependence upon the vehicle speed v inaccordance with Equation (4).

The tracking LPF 55 has a construction identical (except for the factthat the cut-off frequency is different) with that of the LPF (composedof the capacitor 72, voltage-controlled variable resistance element 74and control voltage generating circuit 76) in the tracking BPF 52, andthe cut-off frequency thereof varies in dependence upon the vehiclespeed v.

When the frequency of the low-frequency components to be extracted bythe tracking LPF 55 is set to be 1/10 of the center frequency, thecut-off frequency of this filter should be made 20 v/3.6 (Hz), withreference to Equation (4).

A specific example of the construction of the amplitude detector circuit54 is shown in FIG. 11. The circuit 54 is constituted by a half-waverectifier circuit 77 and a low-pass filter (LPF) 78. A full-waverectifier circuit can also be used instead of the half-wave rectifiercircuit 77. The pass band of the LPF 78 is decided from the standpointof the response time required for road surface detection. By way Ofexample, if the response time is 0.1 (S) and the LPF 78 is a primarylow-pass filter, then the cut-off frequency is 3.7 (Hz).

The output signal of the light sensor 31B (or of the light sensor 31A)is outputted, via a low-pass filter (LPF) 57, as a signal representingamount Dc of diffuse reflected light. The LPF 57 is for removing verylow-frequency undulations contained in the output signal of thephotoelectric detector 31B. The cut-off frequency of this filter is setto about 1 (Hz) (fixed), by way of example.

The output signal of the light sensor 32 for regular reflected light isa signal representing an amount Dd of regular reflected light. Alow-pass filter having an appropriate pass band may be connected to theoutput side of the light sensor 32.

The output signal of the road surface thermometer 33 is a signalrepresenting road surface temperature De. This may be a thermometer(thermosensitive element) for sensing air temperature rather than thetemperature of the road surface. In this case, the thermometer would beprovided at a location in contact with the outside air.

The light source 11 for illuminating the road surface and the lightsource 12 for regular reflection are controlled by automatic powercontrol (APC) circuits 61 and 62, respectively. As a result, the amountof light projected for each of the light sources 11, 12 is held constantat all times.

Fed into a discriminating circuit 60 are the signal outputted by theamplitude detector circuit 54 representing the intensity Da of thecenter-frequency component, the signal outputted by the amplitudedetector circuit 56 representing the intensity Db of the low-frequencycomponent, the signal outputted by the LPF 57 representing the amount Dcof diffuse reflected light, the signal outputted by the light sensor 32representing the amount Dd of regular reflected light, and the signaloutputted by the road surface thermometer 33 representing the roadsurface temperature De.

The discriminating circuit 60 uses two or more of these input signals toidentify or discriminate road surface condition in accordance withroad-surface discrimination algorithms described later. Preferably, thediscriminating circuit 60 is constituted by a CPU (e.g., amicrocomputer), a memory and other periphery circuitry. In such case,the above-mentioned signals Da˜De would be applied to the discriminatingcircuit 60 upon being converted to digital data by an A/D convertercircuit.

(4) Road-surface discrimination algorithm (first aspect thereof)

FIG. 12 illustrates the simplest road-surface discrimination algorithm.Processing in accordance with this road-surface discrimination algorithmis executed in the discriminating circuit 60. This is true for the otherroad-surface discrimination algorithms as well.

The ratio Db/Da of the low-frequency component intensity Db to thecenter-frequency component intensity Da is calculated and the ratio iscompared with the above-mentioned threshold values TH1 and TH2. The roadis judged to be a snowy road if the ratio Db/Da is larger than thethreshold value TH1 (this ratio shall be called "large"); a gravel roadif the ratio is between the threshold values TH1 and TH2 (this ratioshall be called "medium"); and an asphalt road if the ratio is less thanthe threshold value TH2 (this ratio shall be called "small").

Solely the threshold value TH1 may be set in the discriminating circuit60 in advance and only snowy and gravel roads may be discriminated.

Solely the threshold value TH2 (or a suitable value ranging from TH1 toTH2) may be set in the discriminating circuit 60 in advance and onlysnowy and asphalt roads may be discriminated.

Solely the threshold value TH2 may be set in the discriminating circuit60 in advance and only gravel and asphalt roads may be discriminated.

(5) Road-surface discrimination algorithm (second aspect thereof)

FIG. 13 illustrates a road-surface judgment algorithm which further usesthe signal representing the amount Dd of regular reflected light,provided by the light sensor 32, to discriminate whether an asphalt roadis in a wet condition or dry condition.

The road is an asphalt road in a case where the ratio Db/Da is less thanthe threshold value TH2.

In a case where the surface of an asphalt road is wet, the road surfaceis in a state approximating a mirror surface and the amount Dd ofregular reflected light becomes large in comparison with the dry state.A threshold value is set to a level approximately intermediate theamount of regular reflected light obtained when the asphalt road is inthe wet state and the amount of regular reflected light obtained whenthe asphalt road is in the dry state. The condition is judged to be wetasphalt if the amount Dd of regular reflected light is greater than thisthreshold value (this amount shall be referred to as "large"), and dryasphalt if the amount Dd of regular reflected light is less than thisthreshold value (this amount shall be referred to as "small").

Judgment regarding a gravel road and snowy road is the same as thatbased upon the algorithm shown in FIG. 12.

It goes without saying that an arrangement may be adopted in which onlya wet asphalt road and a dry asphalt road are discriminated, judgmentregarding a gravel road can be added to this or judgment regarding asnowy road can be added to this.

(6) Road-surface discrimination algorithm (third aspect thereof)

FIG. 14 is for making a more detailed judgment regarding road surfacecondition by further utilizing the signal outputted by the LPF 57representing the amount Dc of diffuse reflected .light and the signaloutputted by the road surface thermometer 33 representing the roadsurface temperature De.

In general, water freezes at 0 (° C.). Accordingly, if the road surfacetemperature De is 0 (° C.) or less, there is the possibility offreezing. It is determined whether the road surface temperature Deexceeds the freezing temperature (this temperature shall be referred toas "high") or is equal to or less than the freezing temperature (thistemperature shall be referred to as "low").

The freezing temperature need not be exactly 0 (° C.) but may bedetermined to be optimum temperature based upon experience. In a casewhere air temperature is used instead of road surface temperature, airtemperature at which a frozen road is capable of remaining frozenwithout thawing or air temperature at which a road surface starts tofreeze would serve as the threshold value for making judgments.

Since a frozen road surface approximates a mirror surface in the samemanner as a wet road surface, the amount Dd of regular reflected lightis "large".

Accordingly, a road surface is judged to be a frozen road surface if theroad surface temperature De is "low" and the amount Dd of regularreflected light is "large". In this case, the amount Dc of diffusereflected light (described in greater detail later) generally is"small".

A road surface is not a frozen road surface if the road surfacetemperature De is "low" and the amount Dd of regular reflected light is"small". In this case, the road surface condition is judged (to be a dryasphalt road or gravel road) based upon the ratio of low-frequencycomponent intensity Db to the center-frequency component intensity Da.Snow is excluded from this judgment because snow is judged based uponthe amount Dc of diffuse reflected light. However, with regard tojudgment of snow based upon the ratio Db/Da and judgment of snow basedupon the amount Dc of diffuse reflected light, the condition of snowdiffers only slightly (and there are times when the conditions are thesame) and therefore snow may be judged based upon the ratio Db/Da.

New snow and snow a part of which remains white even after beingtraveled upon by passing bodies (vehicles, passersby, etc.) reflectslight diffusely. Since the amount of diffuse reflected light from snowis extremely large in comparison with other road surface conditions,snow and other road surface conditions can be judged based upon theamount Dc of diffuse reflected light. A threshold value for making thisjudgment is set to a level between the amount of diffuse reflected lightat the time of snow and the amount of diffuse reflected light at thetime of other road surface conditions.

Snow is determined in a case where the road surface temperature De is"low" and the amount Dc of diffuse reflected light is greater than thethreshold value (this amount is referred to as "large"). It goes withoutsaying that the threshold value of road surface temperature whenfreezing is determined and the threshold value of road surfacetemperature when snow is determined are made different from each other.

Snow determined based upon the amount Dc of diffuse reflected light issnow all or part of the surface of which is white. By contrast, snowdetermined based upon the ratio Db/Da also includes snow which causes afluctuation in the amount of diffuse reflected light at a period greaterthan that for gravel. This snow is not only white snow but also snowdarkened by being traveled on.

A discrimination algorithm for a case where the road surface temperatureDe is "high" is the same as that shown in FIG. 13.

It goes without saying that only part of the discrimination algorithmshown in FIG. 14 can be used so as to identify or judge only one, two ormore types of road surface conditions from among freezing, snow, gravel,dry asphalt and wet asphalt.

(7) Road-surface discrimination algorithm (fourth aspect thereof)

A road-surface judgment algorithm shown in FIG. 15 is similar to thatshown in FIG. 14. In FIG. 15, gravel and asphalt are distinguished fromeach other based upon the ratio Db/Da. The fact that snow is not judgedbased upon the ratio Db/Da is different from the algorithm shown in FIG.14. The algorithm of FIG. 15 may be considered to be a variation of thealgorithm of FIG. 14.

(8) Electrical configuration (second aspect thereof) of road surfacediscriminator

FIG. 16 illustrates an example of a signal processing circuit to utilizethe output signal of the light sensor 34 (see the optical system ofFIG. 1) for monitoring the amount of projected light from the lightsource 12, which is provided in the proximity of the light source 12 forregular reflected light.

The amount Dd of regular reflected light represented by the outputsignal from the light sensor 32 for regular reflected light is dividedin the arithmetic circuit 58 by the value represented by the outputsignal of the light sensor 34. The result Ddd of division is applied tothe discriminating circuit 60 in place of the amount Dd of regularreflected light. The value Ddd is used instead of Dd in the road-surfacediscrimination algorithm mentioned above.

Thus, even if the light projected from the light source 12 fluctuates(which fluctuation arises from a change in temperature or change withtime), the amount of fluctuation is corrected for by the output of thelight sensor 34 and, as a result, accurate discrimination of roadsurface can be carried out.

In FIG. 16, the light source 12 for regular reflected light is notcontrolled by an APC circuit. However, an arrangement may be adopted inwhich the light source is controlled by an APC circuit.

An arrangement may be adopted in which the amount of projected lightfrom the road-surface illuminating light source 11 is sensed in the samemanner and the output signal of the LPF 57 (which output represents theamount Dc of diffuse reflected light) is corrected based upon the sensedamount of light.

The arithmetic circuit 58 is not limited to a dividing circuit but maybe a subtracting circuit.

Other components (portions deleted from the drawing) of this processingcircuit are the same as those shown in FIG. 8.

(9) Electrical configuration (third aspect thereof) of road surfacediscriminator

FIG. 17 illustrates an example of another arrangement of a signalprocessing circuit so devised as to eliminate the influence ofextraneous light. This is applied to the optical system illustrated inFIGS. 1 through 3.

A pulse oscillator circuit 63 generates a pulse signal having a constantfrequency (described later) and applies the signal to an APC circuit 61Aand dualcomb filter 64. The APC circuit 61A drives the road-surfaceilluminating light source 11 in sync with the applied pulse signal.Accordingly, the light source 11 projects pulsed light toward the roadsurface at a constant period.

The light sensors 31A, 31B receive the pulsed diffuse reflected light sothat the output signal of the differential amplifier circuit 51 alsobecomes a pulsed signal. This signal can be thought of as being oneobtained by pulse-width modulating the diffuse reflected light. Anexample of the output signal from the differential amplifier circuit 51is illustrated in FIG. 19.

The output signal of the differential amplifier circuit 51 possesses awaveform which is the result of superposition of extraneous light uponthe pulsed signal (the peak value whereof represents the amount ofdiffuse reflected light) synchronized to the oscillation pulses from thepulse oscillator circuit 63. The dualcomb filter 64 eliminates theextraneous light and smoothes the pulsed signal.

FIG. 20 illustrates one pulse extracted from FIG. 19 and shown inenlarged form. The level of the signal is sampled and held at a timingSH1 immediately preceding the leading edge of the pulse, a timing SH3immediately following the trailing edge of the pulse and at a time SH2intermediate the leading and trailing edges. Let these levels that aresampled and held be referred to as V1, V2 and V3, respectively. Thelevel of one pulse is calculated in accordance with the next formula byusing these levels and the level is maintained until the next pulse. Theinfluence of the extraneous light is thus eliminated.

    2V2-(V1+V3)                                                Eq. (5)

The dualcomb filter 64 performs the above-mentioned sample-and-holdoperation, arithmetic operation and output maintaining operation. Aspecific example of the construction thereof is illustrated in FIG. 18.

The pulsed signal from the pulse oscillator circuit 63 is applied to atiming generator circuit 87 as a synchronizing signal. The timinggenerator circuit 87 generates timing signals respectively representingthe aforesaid timings SH1, SH2, SH3 and applies the timing signals tosample-and-hold (S/H) circuits 81, 82, 83. The output signal of thedifferential amplifier circuit 51 enters the S/H circuits 81, 82, 83 andis sampled and held at each of the timings.

Outputs V1, V3 of the S/H circuits 81, 83 are added by an adder circuit84, and the output V2 of the S/H circuit 82 is doubled by a doublingcircuit 85. The output of the adder circuit 84 is subtracted from theoutput of the doubling circuit 85 in a subtractor circuit 86 and theresult of subtraction is delivered as an output signal. This outputsignal is held until the calculation regarding the next pulse isfinished. The output signal of the dualcomb filter 64 is applied to thetracking BPF 52 and the tracking LPF 55.

An arrangement may be adopted in which the pulse oscillator circuit 63is provided with the timing generating circuit 87 and a sample-and-holdtiming signal from the pulse oscillator circuit 63 is applied to thedualcomb filter 64.

In the operation described above, delay time and light propagation timein the light source 11 and light sensors 31A, 31B can be ignored. Thesample-and-hold timing is not limited to that set forth in the exampledescribed above. It will suffice if sample-and-hold is performed atleast at two times, namely at the moment the pulse peaks and at a momentbefore the pulse rises or a moment after the pulse decays.

The pulse signal generated by the pulse oscillator circuit 63 has afrequency f_(p) which is required to be no less than twice the maximumfrequency of the input signal to the dualcomb filter 64 (the outputsignal of the differential amplifier circuit 51). That is, the followingis required to hold:

    f.sub.p >2·μ·v.sub.max                Eq. (6)

By way of example, we have f_(p) >11.1 (KHz) if the following hold:μ=0.2 (mm⁻¹) (center spatial frequency) and v_(max) =100 (Km/h)=2.78×10⁴(mm/s) (maximum ground speed).

Other components in FIG. 17 are the same as those shown in FIG. 8.

(10) Electrical configuration (fourth aspect thereof) of road surfacediscriminator

FIG. 21 shows yet another example of the signal processing circuit. Ifthis circuit is compared with the signal processing circuit shown inFIG. 17, it is seen that a sinusoidal oscillator circuit 65 is usedinstead of the pulse oscillator circuit 63 and that the amount ofprojected light from the light source 11 is sinusoidally modulated bythe APC circuit 61B. A heterodyne demodulator (detector) circuit 66 isused instead of the dualcomb filter 64 so that the output signal of thedifferential amplifier circuit 51 is demodulated and then applied to thetracking BPF 52 and the tracking LPF 55. Extraneous light can beeliminated and the S/N ratio improved by this arrangement as well. Othercomponents are the same as those shown in FIGS. 8 and 17.

(11) Electrical configuration (fifth aspect thereof) of road surfacediscriminator

FIG. 22 shows yet a further example of the signal processing circuit.

Both the light source 11 for illuminating the road surface and the lightsource 12 for regular reflected light are driven by pulses. The pulseoscillator circuit 63A generates two types of drives pulses, namely adrive pulse 1 and a drive pulse 2, of different phases. The lightsources 11 and 12 are driven by pulses at different timings by APCcircuits 61A and 62A on the basis of the drive pulses 1 and 2,respectively. A dualcomb filter 67 is provided on the output side of thesensor 32 for regular reflected light in addition to the dualcomb filter64 connected to the output side of the differential amplifier circuit51. The output signal of the dualcomb filter 67 is applied to thediscriminating circuit 60 as a signal representing the amount Dd ofregular reflected light.

Synchronizing signals synchronized to the drive pulses 1 and 2 from thepulse oscillator circuit 36A are applied to the dualcomb filters 64 and67, respectively.

By virtue of the arrangement described above, diffuse reflected lightand regular reflected light can be separated from each other anddetected reliably even if the wavelengths of the projected light fromthe light sources 11 and 12 having the same or nearly the same value.The effects of extraneous light can be eliminated as well.

(12) Optical structure (second aspect thereof) and electricalconfiguration (sixth aspect thereof) of road surface discriminator

FIGS. 23 and 24 illustrate another example of the optical structure ofthe road surface discriminator. Components identical with those shown inFIGS. 1 through 3 are designated by like reference characters and adescription thereof is deleted. Further, a slit plate is indicated bycharacters 22A, an optical filter by characters 23A and a collimatinglens by characters 24A. The light sources 11, 12 and the light sensor 32are not shown in FIG. 24.

FIG. 25 illustrates an electrical configuration (signal processingcircuit) suited to the optical structure shown in FIGS. 23 and 24.Components in this Figure identical with those shown in FIG. 8 are alsodesignated by like reference characters and a redundant explanationthereof is omitted.

The fact that the center-frequency component intensity Da is obtainedbased upon a differential signal between the output signals of the lightsensors 31A and 31B is the same as in the arrangement shown in FIG. 8.Let the light-receiving optical system which includes the light sensors31A and 31B (which system contains the grating plate 25, the prism array26, the condenser lens 27 and the mirror 28) be referred to as a firstlight-receiving optical system.

One other, namely a second, light-receiving optical system is providedto obtain signals respectively representing the low-frequency componentintensity Db and the amount Dc of diffuse reflected light. This secondlight-receiving optical system is constituted by a grating plate 45, acondenser lens 47, a light sensor 41 and a mirror 48.

The light source 11 for illuminating the road surface, thelight-receiving lens 21, the slit plate 22A, the optical filter 23A andthe collimating lens 24A are shared by the first and secondlight-receiving optical systems.

Accordingly, diffuse reflected light from the road surface LD isincident upon not only the first light-receiving optical system but alsothe second light-receiving optical system and is received by the lightsensor 41. The light-reception signal from the light sensor 41 isapplied to the tracking LPF 55 and the LPF 57 after being amplified byan amplifier circuit 42. A signal representing the low-frequencycomponent intensity Db based upon the output signal of the tracking LPF55 and a signal representing the amount Dc of diffuse reflected lightfrom the LPF 57 are obtained.

As mentioned above, the center-frequency component is amplifiedapproximately by a factor of two by taking the difference between theoutput signals of the two light sensors 31A and 31B. However, there areoccasions where some cancellation of the low frequency components occursby taking the aforementioned difference. The other light sensor 41 isprovided to assure the low-frequency components of a higher level.

If necessary, the second light-receiving optical system may be providedwith a spatial filter having a center spatial frequency corresponding tothe low-frequency component to be extracted. In such case it is desiredthat the second light-receiving optical system be provided with twolight sensors and that the output signals of these light sensors beapplied to a differential amplifier circuit.

All of the algorithms described earlier are used as the road-surfacediscrimination algorithms. Further, the approaches taken in themodifications shown in FIGS. 16, 17, 21 and 22 are applicable to thecircuitry of FIG. 25.

(13) Optical structure (third aspect thereof), electrical configuration(seventh aspect thereof) and road-surface discrimination algorithm(fifth, sixth and seventh aspects thereof) of road surface discriminator

FIG. 26 illustrates a further example of the optical structure of theroad surface discriminator. The optical structure is basically the sameas that shown in FIGS. 1 and 2. The differences between two are asfollows:

The light source 12 for regular reflected light, the light sensor 32 forregular reflected light and the sensor 34 for monitoring the amount ofprojected light shown in FIGS. 1 and 2 (and in FIG. 3) are deleted.Instead, a light source 17 for regular reflected light is provided.

The light source 17 for regular reflected light is provided and placedin such a manner that, of the light projected upon the road surface LDfrom the light source 17, the regular reflected light from the roadsurface LD will impinge upon the light sensors 31A, 31B through thelight-receiving lens 21, slit plate 22, optical filter 23, collimatinglens 24, grating plate (slit array) 25, prism array 26 and condenserlens 27. Since the regular reflected light is large in quantity if theroad surface is wet or frozen, the light source 17 need not produce asmuch light as the light source 11 (it will suffice if the number oflight-emitting diodes constructing the light source is small).

FIG. 27 illustrates a signal processing circuit suited to the opticalsystem shown in FIG. 26. This circuit resembles the arrangement shown inFIG. 22 in that the light sources are driven by two types of pulseshaving different timings and the light-reception signal is separated bya dualcomb filter. Components in this Figure identical with those shownin FIG. 22 are designated by like reference characters and a redundantexplanation thereof is omitted; only the differences will be described.

The light source 11 for illuminating the road surface and the lightsource 17 for regular reflected light are driven by pulses alternatelyby the APC circuits 61A, 62A based upon two types of drive pulses 1 and2 having different timings.

Diffuse reflected light and regular reflected light from the roadsurface LD is sensed by the light sensors 31A, 31B and a differentialoutput between the output signals of the two sensors 31A, 31B isobtained in the differential amplifier circuit 51. The output of thedifferential amplifier circuit 41 is applied to the two dualcomb filters64, 67.

The dualcomb filter 64 operates in sync with pulses driving the lightsource 11 and extracts a signal based upon the diffuse reflected light.Signals respectively representing the center-frequency componentintensity (diffuse reflected light) Da and the low-frequency componentintensity (diffuse reflected light) Db are obtained based upon theoutput signal of the dualcomb filter 64.

The dualcomb filter 67 operates in sync with pulses driving the lightsource 17 and extracts a signal based upon the regular reflected light.The output signal of the filter 67 is applied to a tracking BPF 68. Thetracking BPF 68 is the same as the tracking BPF 52 and receivesfeedback, namely the voltage signal representing the speed v. The outputsignal of the tracking BPF 68 is applied to an amplitude detectorcircuit 69. A signal representing the center-frequency componentintensity (regular reflected light) Df is obtained from the amplitudedetector circuit 69 and is applied to the discriminating circuit 60.

The light sources 11 and 17 can be modulated by signals having differentfrequencies and the outputs of the differential amplifier 51 can bedemodulated by the modulation frequencies, whereby it is also possibleto achieve separation into a signal based upon diffuse reflected lightand a signal based upon regular reflected light.

FIGS. 28, 29 and 30 illustrate road-surface discrimination algorithms(fifth, sixth and seventh aspects thereof), respectively. Thesealgorithms basically are the same as those shown in FIGS. 13, 14 and 15.The single difference is as follows:

Specifically, the amount Dd of regular reflected light used in thealgorithms of FIGS. 13 through 15 is replaced by the ratio Df/Da of thecenter-frequency component intensity (regular reflected light) Df to thecenter-frequency component intensity (diffuse reflected light) Da. Sincethe amount of regular reflected light becomes large under freezing orwet conditions, the ratio Df/Da becomes large under these conditions.Accordingly, a condition can be judged to be a freezing or wet conditionby discriminating the ratio Df/Da using a suitable threshold value.

(14) Electrical configuration (eighth, ninth, tenth, 11th and 12thaspects thereof) of road surface discriminator

FIG. 31 illustrates an example (eight aspect) of a simplified processingcircuit in the road surface discriminator. An example will be describedin which a road surface condition is distinguished to be either snowy orgravel. Components in this Figure already described are designated bylike reference characters and a redundant explanation thereof isomitted.

The output signal of the tracking BPF 52 contains a center-frequencycomponent. This output signal is binarized in a binarizing circuit 81using the zero level thereof as a threshold-value level, and thebinarized signal enters a counter 82 (referred to as "counter A").

On the other hand, the output signal of the differential amplifiercircuit 51 contains low-frequency components and not only acenter-frequency component. As described with reference to FIG. 7, thelevels of the low-frequency components change in dependence upon theroad surface condition and are ranked as follows in order of decreasinglevel: snow, gravel and asphalt. This signal is binarized in abinarizing circuit 83 using the zero level thereof as a threshold-valuelevel, and the binarized signal enters a counter 84 (referred to as"counter B").

FIGS. 32 and 33 illustrate the waveforms of the output from thedifferential amplifier circuit 51, the output from the binarizingcircuit 83, the output of the tracking BPF 52 and the output of thebinarizing circuit 81, with FIG. 32 being for the case of a snowy roadsurface and FIG. 33 being for the case of a gravel road surface.

Since the output signal of the tracking BFP 52 is composed mainly ofcenter-frequency components, the counter A counts a substantiallyconstant value (speed is assumed to be constant) for a fixed period oftime irrespective of the road surface condition.

By contrast, the output signal of the differential amplifier circuit 51contains low-frequency components in addition to the center-frequencycomponents. Since the amplitude of the low-frequency components is highin case of a snowy road, the center-frequency components assume a formin which they are superposed upon the low-frequency components.Accordingly, the number of pulses formed in a case where this signal isbinarized is small and the value of the count in the counter B duringthe fixed period of time is small.

In a case where the road surface is gravel, the amplitude of thelow-frequency components is small. Hence, the number of pulses formed ina case where the output signal of the differential amplifier circuit 51is binarized at the zero level approaches the number of pulses obtainedin a case where the center-frequency components are binarized. The valueof the count in the counter B during the fixed period of time becomes avalue near the value of the count in the counter A.

Accordingly, by comparing (as by taking the ratio of) the values of thecounts in counter A and counter B in the fixed period of time, the roadsurface condition can be discriminated to be snow or gravel.

A timing circuit 86 resets the counters A, B and causes them to startcounting the input pulses at fixed times. A discriminating circuit 85reads the values of the counts in the counters A, B immediately beforethe counters A, B are reset and judges the road surface condition basedupon the ratio between the counts.

In accordance with this approach, it is possible to distinguish betweensnow and asphalt, between asphalt and gravel, and among snow, asphaltand gravel.

It goes without saying that by taking into consideration the amount ofregular reflected light, the amount of diffuse reflected light and theroad surface temperature, as described above, it is possible todiscriminate wetness, freezing and snow, etc., as well.

FIG. 34 illustrates yet another example (a ninth aspect) of the signalprocessing circuit. In the example depicted in FIG. 31, the road surfaceis discriminated by utilizing the ratio between the values of the countsin the two counters A, B over a fixed period of time. In the circuitillustrated in FIG. 34, a discrimination output is obtained whenever thevehicle travels a fixed distance.

More specifically, the value of the count in the counter B at the momentthe value of the count in the counter A reaches a predeterminedprescribed value is read into the discriminating circuit 85. The latterdiscriminates the road surface condition based upon the value of thecount in the counter B. The calculation of the ratio between the valuesof the counts in counters A, B by the discriminating circuit 85 can beeliminated.

When the value of the count in counter A attains the predeterminedprescribed value, the counters A, B are reset and then start countingagain.

By way of example, assume that the center spatial frequency of the roadsurface selected by the spatial filter is 0.25 (mm⁻¹) [the period is 4(mm)]. Let the prescribed value set in the counter A be 250. In thiscase, road surface discrimination is performed a single time wheneverthe vehicle travels 4 (mm)×250=1 (m).

In the processing circuit shown in FIG. 31, the road surface conditionis judged based upon the values of the counts in the counters A, B inthe fixed period of time. Therefore, when the traveling speed of thevehicle is low, the values of the counts in the counters A, B are smalland there are occasions where the discrimination accuracy isunsatisfactory. With the processing circuit shown in FIG. 34, the roadsurface is discriminated at the fixed traveling distances of thevehicle. As a result, response time is lengthened when the vehicletraveling speed is low.

A signal processing circuit (tenth aspect thereof) shown in FIG. 35 isan improvement over the processing circuit of FIG. 31 and is providedwith four sets of counters A and B (counters A1 and B1; A2 and B2; A3and B3; and A4 and B4). As shown in FIG. 36, the counters A, B of eachset are reset/started at short time intervals under the control of thetiming control circuit 86A. For example, if the counting time of thecounters A, B of each set is 0.8 sec, the counting operation of thecounters A, B in each set is started every 0.2 sec. Multiplexers 87, 88change over the set of counters every 0.2 sec and apply the values ofthe counts to the discriminating circuit 85. As a result, the roadsurface discrimination period is 0.2 sec. Even if the road surfacediscrimination period is short, the counting time of the counters ineach set is long (0.8 sec) and, hence, there is no decline indiscrimination accuracy.

A signal processing circuit (11th aspect thereof) shown in FIG. 37 is animprovement over the processing circuit shown in FIG. 34.

The counting operation of each set of counters A and B starts independence upon the value of the count in counter A1. For example, let256 be the value of the count in set in counters A. When the value ofthe count in counter A1 is zero, the counters A1 and B1 start counting.When the values of counts registered by counter A1 are 64, 128 and 192,counters A2 and B2, counters A3 and B3 and counters A4 and B4,respectively, start counting.

When the value of the count in counter A1 reaches 256, the value of thecount in counter B1 is applied to the discriminating circuit 85 via themultiplexer 88. Similarly, when the values of the counts in counters A2,A3 and A4 each reach 256, the values of the counts in the counters B2,B3 and B4 enter the discriminating circuit 85 via the multiplexer 88.

Even if the set counts in counters A are the same, the period of roadsurface discrimination takes on a value of one divided by the number ofcounter sets and response time is thus shortened over that of thecircuitry shown in FIG. 34.

FIG. 38 illustrates an example (12th aspect) in which the amplificationfactor of the differential amplifier circuit contained in the signalprocessing circuit and of other amplifier circuits is controlled inconformity with the results of road surface discrimination.

In case of a snowy road surface, for example, the light-receivingcircuitry may saturate because the amount of diffuse reflected light islarge. In order to prevent this situation from occurring, the gain ofthe amplifier circuitry is lowered when snow is discriminated.

A circuit (e.g., a feedback resistor) 91 which decides an effectiveamplification factor at the time of a road surface condition other thansnow and a circuit 92 which provides a lower amplification factor usedin case of snow are connected to the differential amplifier circuit 51via analog switches 93 and 94, respectively. Control is performed by again control circuit 96 in dependence upon the results of road surfacediscrimination by the discriminating circuit 85 in such a manner thatthe analog switch 94 is turned on at the time of snow and the analogswitch 93 is turned on at all other times. An inverter is indicated at95. Saturation of the light-receiving circuitry can thus be prevented.

When gain control is performed based upon the reflectivity of the roadsurface, gain declines in case of a painted road surface (thereflectivity of which is high). Though the reflectivity of a paintedroad surface is high, the amplitude of the output from the light sensorof a spatial filter does not become very large. With the circuit of FIG.38, gain will not be lowered erroneously even in case of such a roadsurface.

In the case of a snowy road, the amount of projected light from thelight source 11, etc., may be reduced instead of lowering the gain ofthe amplifier circuitry.

(15) Example of applications of road surface discriminator

FIG. 39 illustrates an example of an apparatus for providing a driverwith a necessary warning in dependence upon the results of road surfacediscrimination.

An automobile is equipped with a warning apparatus 102 which warns thedriver of the fact that the possibility of slipping is high when sand orgravel, snow or freezing is detected by a road surface discriminator 100having the above-described construction mounted on the vehicle. Theresult of road surface discrimination by the road surface discriminator100 is applied to an ECU (electronic control unit) 101 for an ABS(antilock brake system). When the ECU 101 has judged that thepossibility of slipping is high (the above-mentioned snow or freezingcondition), the ECU 101 outputs a signal to the warning apparatus 102 toso notify the driver of this fact. This warning is realized by an audiooutput, a display on a screen or lighting of an indicator lamp.

FIG. 40 illustrates the construction of an ABS equipped with a roadsurface discriminator for outputting speed and the result of roadsurface discrimination.

The outputs (ground speed v and result of road surface discrimination)of the road surface discriminator 100 and the output (wheel speed v_(w))of a wheel rotational-speed sensor 103 are applied to the ECU 101.

The wheel rotational-speed sensor 103 comprises a rotor 104 attached toa tire of the vehicle and having a number of magnetic-pole teeth on thecircumference thereof, and an electromagnetic pick-up-type sensor 105for outputting a pulsed signal whose frequency is proportional to therotational speed of the rotor 104. The frequency of the output pulsesfrom the sensor 105 represents the wheel speed v_(w).

The ECU 101 calculates slip factor λ in accordance with the followingequation based upon the input ground speed v and wheel speed v_(w) :

    λ=(v-v.sub.w)/v                                     Eq. (7)

The ECU 101 further includes a table of the kind shown in FIG. 41storing optimum slip factors λ_(m) that conform to the road surfaceconditions. This table is created by preliminary testing.

The ECU 101 reads the optimum slip factor λ_(m) conforming to the resultof road surface discrimination out of the table and outputs abrake-pressure control signal so as to make the calculated slip factor λand the optimum slip factor λ_(m) coincide. This brake controlpreferably is carried out feedback control.

FIG. 42 illustrates an automobile equipped with the above-mentioned ABS.The ABS is constructed to minimize traveling distance on all roadsurfaces, without the tires locking up, by executing the above-describedprocessing based upon an output signal from the road surfacediscriminator 100, which has a ground-speed sensing function, and anoutput signal from the wheel rotational-speed sensor 103.

Further, a traction control system (TCL) for controlling accelerationcan be constructed in such a manner that tires will not slip at themoment of forward propulsion, this being accomplished by storing theresult of road surface discrimination immediately before the vehicle isstopped. In TCL, a table in which an optimum engine rpm for each type ofroad surface has been stored is created in advance. In accordance withthe result of road discrimination stored just prior to stopping of thevehicle, the ECU 101 refers to the table and acceleration is controlledin such a manner that the optimum engine rpm commensurate with the roadsurface condition will be attained.

Whether or not forward propulsion of the vehicle has been performednormally is judged by the ECU 101 based upon ground speed. If thevehicle is slipping, the ECU 101 performs control so as to lower enginerpm.

FIG. 43 illustrates a suspension control apparatus having the roadsurface discriminator, and FIG. 44 is a sectional view of a shockabsorber, which is the principal component used in suspension control.

The road surface discriminator 100 provides the ECU 101 with the resultof road surface discrimination and the ground speed v. The ECU 101provides a shock absorber 106 with a control signal.

In order to assure vehicle maneuverability, safety and riding comfortregardless of road surface conditions, it is required that so-called"tightness" of the suspension be controlled in conformity with the roadsurface condition. For example, suspension is relaxed in case of agravel or sandy road the surface of which highly uneven and is tightenedin case of an asphalt or concrete road.

A function of this kind is implemented by controlling, in the mannerdescribed below, the degree of opening of electromagnetic valves withinthe shock absorber in dependence upon the result of road surfacediscrimination.

As shown in FIG. 44, a piston 115 secured to the chassis of anautomobile moves up and down within a cylinder 111, which is secured tothe frame of the automobile, as the chassis oscillates. When the piston115 moves, a fluid (oil) filling hydraulic chambers 112A, 112B movesbetween the hydraulic chambers 112A, 112B through passageways ofelectromagnetic valves 116. The electromagnetic valves 116 are drivenand controlled by a control signal from the ECU 101. As a result, thearea of the fluid passageway in each electromagnetic valve 116 changes.This makes it possible to control the vibration attenuating force.

When the road surface discriminator 100 has discriminated a gravel orsandy road surface, the electromagnetic valves 116 are opened widely torelax the suspension. When the road surface has been discriminated asbeing asphalt or concrete, the electromagnetic valves 116 are openednarrowly to tighten the suspension.

A free piston 114 and an air chamber 113 are for the purpose ofabsorbing a difference in the change of volume in the hydraulic chambers112A, 112B. Control for opening and closing the electromagnetic valves116 may use the speed v and not only the result of road surfacediscrimination. For example, at high traveling speed the shock receivedis large even when a road surface is only slightly irregular. As suchtime, therefore, control should be performed so as to relax thesuspension.

What is claimed is:
 1. A road surface discriminator comprising:a firstlight source for projecting light toward a road surface; a first spatialfilter light-receiving optical system having first spatial filter meansand first light-receiving means, wherein reflected light from the roadsurface on which light is projected from said first light source isreceived by the first light-receiving means through said first spatialfilter means and the first-light receiving means outputs an electricsignal representing the reflected light received; a secondlight-receiving optical system having second light-receiving means,wherein reflected light from the road surface on which light isprojected from said first light source is received by said secondlight-receiving means and said second-light receiving means outputs anelectric signal representing the reflected light received; a firstsignal processing circuit for detecting intensity of a center-frequencycomponent, which corresponds to the spatial frequency of said firstspatial filter means, based upon the electric signal outputted by saidfirst spatial filter light-receiving optical system; a second signalprocessing circuit for detecting intensity of a low-frequency component,which corresponds to a spatial frequency lower than the spatialfrequency of said first spatial filter means, based upon the electricsignal outputted by said second light-receiving optical system; anddiscriminating means for discriminating condition of the road surfacebased upon the intensity of the center-frequency component detected bysaid first signal processing circuit and the intensity of thelow-frequency component detected by the second signal processingcircuit.
 2. A road surface discriminator according to claim 1, whereinsaid second light-receiving optical system is contained in said firstspatial filter light-receiving optical system and said secondlight-receiving means is said first light-receiving means;the electricsignal outputted by said first light-receiving means being applied tosaid first signal processing circuit and said second signal processingcircuit.
 3. A road surface discriminator according to claim 1, whereinpart of said second light-receiving optical system is contained in saidfirst spatial filter light-receiving optical system, and said secondlight-receiving means receives reflected light that does not passthrough said first spatial filter means.
 4. A road surface discriminatoraccording to claim 1, wherein said second light-receiving optical systemhas a second spatial filter having a spatial frequency lower than thespatial frequency of said first spatial filter means, and said secondlight-receiving means receives reflected light through said secondspatial filter.
 5. A road surface discriminator according to claim 1,wherein a road surface condition discriminated by said discriminatingmeans is at least one of snow, gravel and asphalt.
 6. A road surfacediscriminator according to claim 1, wherein said first light source,said first spatial filter light-receiving optical system and said secondlight-receiving optical system are arranged in such a manner that saidfirst and second light-receiving means receive diffuse reflected lightfrom the road surface.
 7. A road surface discriminator according toclaim 1, further comprising a second light source for projecting lighttoward the road surface and a third light-receiving optical system,which includes a third light-receiving element, the second light sourceand the third light-receiving optical system being arranged in such amanner that said third light-receiving element receives regularreflected light from the road surface on which light is projected fromsaid second light source;said discriminating means discriminatingcondition of wetness of the road surface based upon an output signalfrom said third light-receiving element.
 8. A road surface discriminatoraccording to claim 7, further comprising a light-quantity detector fordetecting the quantity of light in the projected light from said secondlight source;said discriminating means discriminating the condition ofwetness of the road surface based upon a value obtained by normalizingthe output signal of said third light-receiving element by the quantityof light detected by said light-quantity detector.
 9. A road surfacediscriminator according to claim 7, wherein said third light-receivingoptical system is contained in said first spatial filter light-receivingoptical system and said third light-receiving element is said firstlight-receiving element.
 10. A road surface discriminator according toclaim 9, further comprising:separating means for separating an outputsignal of said first light-receiving element into a first signalcomponent, which results from the projected light of said first lightsource, and a second signal component, which results from the projectedlight of said second light source; said first signal processing circuitdetecting intensity of the center-frequency component of diffused lightbased upon said first signal component separated by said separatingmeans; and a third signal processing circuit for detecting intensity ofthe center-frequency component of regular reflected light based uponsaid second signal component separated by said separating means; saiddiscriminating means discriminating the condition of wetness of the roadsurface based upon a ratio of said intensity of the center-frequencycomponent of regular reflected light to said intensity of thecenter-frequency component of diffused light.
 11. A road surfacediscriminator according to claim 10, further comprising:a drive circuitfor driving said first light source and said second light source basedupon respective signals of different phase or different frequency; andsaid separating means being a demodulating circuit for demodulatingrespective signals that have been modulated by signals of differentphase or signals of different frequency.
 12. A road surfacediscriminator according to claim 7, further comprising a temperaturesensor for measuring road surface temperature or air temperature;saiddiscriminating means determining freezing if, when wetness has beendiscriminated, the temperature sensed by said temperature sensor is lessthan a predetermined value.
 13. A road surface discriminator accordingto claim 7, further comprising:a drive circuit for driving said firstand second light sources based upon respective signals of differentphase or different frequency; and a demodulating circuit fordemodulating output signals from said first and second light-receivingmeans and an output signal from third light-receiving means.
 14. A roadsurface discriminator according to claim 7, further comprising controlmeans for varying the quantity of projected light from said first orsecond light source in dependence upon result of discriminationperformed by said discriminating means.
 15. A road surface discriminatoraccording to claim 1, further comprising a fourth signal processingcircuit for detecting a diffused light component of very low frequencybased upon the output signal of said first light-receiving element orsaid second light-receiving element;said discriminating means judgingsnow in a case where the diffused light component detected by the fourthsignal processing means exceeds a predetermined value.
 16. A roadsurface discriminator according to claim 1, wherein said first lightsource has:a plurality of light-emitting elements; and a plurality oflenses corresponding to said light-emitting elements and disposed infront of them; optic axes of said light-emitting elements and optic axesof the lenses corresponding thereto being offset from each other.
 17. Aroad surface discriminator according to claim 1, wherein said firstsignal processing circuit includes a tracking band-pass filter, saidtracking band-pass filter outputting a signal representing ground speedand the pass band thereof being controlled based upon the speed signal.18. A road surface discriminator according to claim 17, wherein saidsecond signal processing circuit includes a tracking low-pass filter inwhich cut-off frequency is controlled based upon said speed signal. 19.A road surface discriminator according to claim 1, further comprising:adrive circuit for driving said first light source based upon a signalwhich varies with time; and a demodulating circuit for demodulatingoutput signals from said first and second light-receiving means.
 20. Aroad surface discriminator according to claim 1, further comprising:anamplifier circuit for amplifying electric signals inclusive of alight-reception signal; and means for controlling amplification factorof said amplifier circuit in dependence upon result of discriminationperformed by said discriminating means.
 21. A road surface discriminatoraccording to claim 1, further comprising a display device for displayingresult of discrimination performed by said discriminating means.
 22. Aroad surface discriminator according to claim 1, further comprising aspeed detecting circuit for outputting a signal representing groundspeed based upon said center-frequency component.
 23. A vehicle in whichthe road surface discriminator set forth in claim 22 has been installed.24. A vehicle according to claim 23, further comprising:a wheel speedsensor for sensing wheel speed; and brake control means for controllinga brake based upon result of road surface discrimination and groundspeed provided by said road surface discriminator and wheel speedprovided by said wheel speed sensor.
 25. A vehicle according to claim23, further comprising:a shock absorber provided on a vehicle body; andsuspension control means for controlling said shock absorber independence upon result of road surface discrimination and ground speedfrom said road surface discriminator.
 26. A road surface discriminatorcomprising:a source for projecting light toward a road surface; aspatial filter light-receiving optical system for receiving diffusereflected light from the road surface, on which light is projected fromsaid light source, through a spatial filter and outputting an electricsignal representing the reflected light received; a first signalprocessing circuit for detecting intensity of a center-frequencycomponent, which corresponds to the spatial frequency of the spatialfilter, based upon the electric signal outputted by said spatial filterlight-receiving optical system; a second signal processing circuit fordetecting intensity of a low-frequency component, which corresponds to aspatial frequency lower than the spatial frequency of said spatialfilter means, based upon the electric signal outputted by said spatialfilter light-receiving optical system; and discriminating means fordiscriminating condition of the road surface based upon the intensity ofthe center-frequency component detected by said first signal processingcircuit and the intensity of the low-frequency component detected by thesecond signal processing circuit.
 27. A road surface discriminatoraccording to claim 26, wherein a road surface condition discriminated bysaid discriminating means is at least one of snow, gravel and asphalt.28. A road surface discriminator according to claim 26, furthercomprising a second light source for projecting light toward the roadsurface and a second light-receiving optical system, which includes asecond light-receiving element, the second light source and the secondlight-receiving optical system being arranged in such a manner that saidsecond light-receiving element receives regular reflected light from theroad surface on which light is projected from said second lightsource;said discriminating means discriminating condition of wetness ofthe road surface based upon an output signal from said secondlight-receiving element.
 29. A road surface discriminator according toclaim 28, further comprising a temperature sensor for measuring roadsurface temperature or air temperature;said discriminating meansdetermining freezing if, when wetness has been discriminated, thetemperature sensed by said temperature sensor is less than apredetermined value.
 30. A road surface discriminator comprising:a lightsource for projecting light toward a road surface; a spatial filterlight-receiving optical system for receiving diffuse reflected lightfrom the road surface, on which light is projected from said lightsource, through a spatial filter and outputting an electric signalrepresenting the reflected light received; a signal processing circuitfor detecting a center-frequency component, which corresponds to thespatial frequency of said spatial filter, based upon the electric signaloutputted by said spatial filter light-receiving optical system; firstcounting means for counting zero cross points of the center-frequencycomponent outputted by said signal processing circuit; second countingmeans for counting zero cross points of the electric signal outputted bysaid spatial filter light-receiving optical system; and discriminatingmeans for discriminating condition of the road surface based upon acount value in said first counting means and a count value in saidsecond counting means.
 31. A road surface discriminator according toclaim 30, wherein a road surface condition discriminated by saiddiscriminating means is at least one of snow, gravel and asphalt.
 32. Aroad surface discriminator according to claim 30, wherein saiddiscriminating means performs discrimination and said first and secondcounting means start counting at prescribed times.
 33. A road surfacediscriminator according to claim 30, wherein said discriminating meansperforms discrimination and said first and second counting means startcounting whenever a count in said first counter reaches a prescribednumber.
 34. A road surface discriminator according to claims 30, whereinsaid first and second counting means are provided in a plurality ofpairs, and the moment at which counting by said first and secondcounting means starts is set so as to be different for each pair.
 35. Aroad surface discriminator according to claims 30, further comprising asecond light source for projecting light toward the road surface and asecond light-receiving optical system, which includes a secondlight-receiving element, the second light source and the secondlight-receiving optical system being arranged in such a manner that saidsecond light-receiving element receives regular reflected light from theroad surface on which light is projected from said second lightsource;said discriminating means discriminating condition of wetness ofthe road surface based upon an output signal from said secondlight-receiving element.
 36. A road surface discriminator according toclaim 35, further comprising a temperature sensor for measuring roadsurface temperature or air temperature;said discriminating meansdetermining freezing if, when wetness has been discriminated, thetemperature sensed by said temperature sensor is less than apredetermined value.
 37. An object discriminator comprising:a firstlight source for projecting light toward a surface of the object; afirst spatial filter light-receiving optical system having first spatialfilter means and first light-receiving means, wherein reflected lightfrom the surface of the object on which light is projected from saidfirst light source is received by the first light-receiving meansthrough said first spatial filter means and the first-light receivingmeans outputs an electric signal representing the reflected lightreceived; a second light-receiving optical system having secondlight-receiving means, wherein reflected light from the surface of theobject on which light is projected from said first light source isreceived by said second light-receiving means and said second-lightreceiving means outputs an electric signal representing the reflectedlight received; a first signal processing circuit for detectingintensity of a center-frequency component, which corresponds to thespatial frequency of said first spatial filter means, based upon theelectric signal outputted by said first spatial filter light-receivingoptical system; a second signal processing circuit for detectingintensity of a second frequency component, which corresponds to aspatial frequency lower or higher than the spatial frequency of saidfirst spatial filter means, based upon the electric signal outputted bysaid second light-receiving optical system; and discriminating means fordiscriminating condition of the surface of the object based upon theintensity of the center-frequency component detected by said firstsignal processing circuit and intensity of the second frequencycomponent detected by the second signal processing circuit.
 38. Anobject discriminator comprising:a first light source for projectinglight toward a surface of the object; a spatial filter light-receivingoptical system for receiving diffuse reflected light from the surface ofthe object, on which light is projected from said light source, througha spatial filter and outputting an electric signal representing thereflected light received; a first signal processing circuit fordetecting intensity of a center-frequency component, which correspondsto the spatial frequency of the spatial filter, based upon the electricsignal outputted by said spatial filter light-receiving optical system;a second signal processing circuit for detecting intensity of a secondfrequency component, which corresponds to a spatial frequency lower orhigher than the spatial frequency of said spatial filter, based upon theelectric signal outputted by said spatial filter light-receiving opticalsystem; and discriminating means for discriminating condition of thesurface of the object based upon the intensity of the center-frequencycomponent detected by said first signal processing circuit and intensityof the second frequency component detected by the second signalprocessing circuit.
 39. An object discriminator comprising:a lightsource for projecting light toward a surface of the object; a spatialfilter light-receiving optical system for receiving diffuse reflectedlight from the surface of the object, on which light is projected fromsaid light source, through a spatial filter and outputting an electricsignal representing the reflected light received; a signal processingcircuit for detecting a center-frequency component, which corresponds tothe spatial frequency of said spatial filter, based upon the electricsignal outputted by said spatial filter light-receiving optical system;first counting means for counting zero cross points of thecenter-frequency component outputted by said signal processing circuit;second counting means for counting zero cross points of the electricsignal outputted by said spatial filter light-receiving optical system;and discriminating means for discriminating condition of the surface ofthe object based upon a count value in said first counting means and acount value in said second counting means.
 40. A mobile body having aroad surface discriminator, the road surface discriminator comprising:afirst light source for projecting light toward a road surface; a firstspatial filter light-receiving optical system having first spatialfilter means and first light-receiving means, wherein reflected lightfrom the road surface on which light is projected from said first lightsource is received by the first light-receiving means through said firstspatial filter means and the first-light receiving means outputs anelectric signal representing the reflected light received; a secondlight-receiving optical system having second light-receiving means,wherein reflected light from the road surface on which light isprojected from said first light source is received by said secondlight-receiving means and said second-light receiving means outputs anelectric signal representing the reflected light received; a firstsignal processing circuit for detecting intensity of a center-frequencycomponent, which corresponds to the spatial frequency of said firstspatial filter means, based upon the electric signal outputted by saidfirst spatial filter light-receiving optical system; a second signalprocessing circuit for detecting intensity of a low-frequency component,which corresponds to a spatial frequency lower than the spatialfrequency of said first spatial filter means, based upon the electricsignal outputted by said second light-receiving optical system; anddiscriminating means for discriminating condition of the road surfacebased upon the intensity of the center-frequency component detected bysaid first signal processing circuit and the intensity of thelow-frequency component detected by the second signal processingcircuit.
 41. A vehicle having a road surface discriminator, the roadsurface discriminator comprising:a first light source for projectinglight toward a road surface; a first spatial filter light-receivingoptical system having first spatial filter means and firstlight-receiving means, wherein reflected light from the road surface onwhich light is projected from said first light source is received by thefirst light-receiving means through said first spatial filter means andthe first-light receiving means outputs an electric signal representingthe reflected light received; a second light-receiving optical systemhaving second light-receiving means, wherein reflected light from theroad surface on which light is projected from said first light source isreceived by said second light-receiving means and said second-lightreceiving means outputs an electric signal representing the reflectedlight received; a first signal processing circuit for detectingintensity of a center-frequency component, which corresponds to thespatial frequency of said first spatial filter means, based upon theelectric signal outputted by said first spatial filter light-receivingoptical system; a second signal processing circuit for detectingintensity of a low-frequency component, which corresponds to a spatialfrequency lower than the spatial frequency of said first spatial filtermeans, based upon the electric signal outputted by said secondlight-receiving optical system; and discriminating means fordiscriminating condition of the road surface based upon the intensity ofthe center-frequency component detected by said first signal processingcircuit and the intensity of the low-frequency component detected by thesecond signal processing circuit.
 42. A vehicle according to claim 41,further comprising:judging means for judging that possibility ofslipping is high based upon result of road surface discriminationperformed by said road surface discriminator; and a warning device forissuing a warning indicative of result of judgment rendered by saidjudging means.
 43. A vehicle according to claim 41, further comprisingmeans for controlling an accelerator based upon result of road surfacediscrimination provided by said road surface discriminator.
 44. Avehicle according to claim 41, comprising:a shock absorber provided on avehicle body; and suspension control means for controlling said shockabsorber in dependence upon result of road surface discriminationperformed by said road surface discriminator.